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62 p r a c t i c a l w i n e r y & v i n e ya r d M a r c H 2 0 1 4 g r a p e g r o w i n g the soil moisture content and, as a consequence, the availability of K to decline. This could be problematic in soils with low K concentrations and could result in a deficiency. If the soil has a high natural K content or has been fertilized recently, then soil water deficit is less likely to cause a deficiency. This model also predicts that excess K uptake is most likely to occur when soil K and soil moisture levels are high. 9 In addition, if deficit irrigation is not going to be practiced and grapevines are going to be irrigated close to 100% of calculated vine evapo- transpiration, then K availability will be optimum and applica- tions of K fertilizer may not be necessary. Whether or not to apply K fertilizer is an important decision because fruit and wine quality may be negatively impacted by excess fruit and wine K levels, and associated high wine pH after vinification. Potassium partitioning in grapevines Studies with Cabernet Sauvignon have shown that the fruit on the vine is the major sink for K in grapevines. 10 Potassium is the major cation in fruit at harvest, and during ripening both the pulp and skin accumulate K in large amounts. 4,5 The high demand for K by the fruit during ripening after véraison is often accompanied by a decrease in petiole, leaf or stem K during this time (see Table 1). 6,10 Potassium, in contrast to other nutrients like Ca or Mg, is not a major component of any plant compounds and, as such, is con- sidered a mobile plant nutrient. It moves primarily in the phloem of the plant, and K accumulation in fruit mirrors sugar accumu- lation. 5 During ripening, therefore, the K demand by fruit appears partially to be supplied by K from leaves and petioles. If the availability of soil K is reduced during ripening and demand for K by fruit is great, then K efflux from leaves could be excessive, leading to expression of characteristic foliar symp- toms of K deficiencies — chlorosis and necrosis of leaf margins, which usually occur late in the season during ripening on basal leaves. The normal time for K deficiencies to begin to appear are shortly after véraison in basal leaves. Given that fruit is the major sink for K uptake, vineyard manag- ers should expect that the demand for potassium will be increased with larger crop load. A deficiency is most likely to occur when the supply of K from soil and foliage is insufficient to keep up with demand. It should not be surprising that K deficiency symp- toms are more noticeable during years with higher than normal yields and lower than normal rainfall, as in 2012 and 2013. Potassium as a factor in fruit, must and wine quality A K deficiency could be associated with delayed sugar accumu- lation in fruit (Table IV), though not always. 9 As potassium SHuR FARMS ® Frost Protection Award Winning Technology! Internationally Acclaimed! Shur Farms Frost Protection 1890 N. 8th St., Colton, CA 92324 Toll Free (877) 842-9688 info@shurfarms.com ~ www.shurfarms.com Cold Air Drain ® Powerful Cost Effective Versatile Convenient Manufacturer of the world's most advanced frost protection You focus on making great wine... Let us take care of the cooling. ProChiller.com 800-845-7781 PACKAGED GLYCOL CHILLER SYSTEMS Visit Our Booth at the Eastern Wine Expo 2014. Véraison Mid-matura on Harvest 1,524 1,467 1,519 1,394 1,219 1,569 * * ns 2.79 3.19 3.25 2.72 3.05 3.17 * * ns 14.5 5.28 3.29 15.9 5.21 3.92 * * ns 13.9 20.2 22.7 12.9 19.7 21.4 * ns * Potassium (mg/L) Irrigated Non-Irrigated Significance Fruit pH Irrigated Non-Irrigated Significance Titratable Acidity (g/L H 2 SO 4 ) Irrigated Non-Irrigated Significance Soluble Solids (Brix) Irrigated Non-Irrigated Significance Table IV. Fruit composition of irrigated and non-irrigated Cabernet Sauvignon grapevines in Somontano (Spain) between véraison and harvest. 6 Ns, * Non-significant and significant at P = 0.05 respec vely